External flue heat exchangers

ABSTRACT

A heat exchanger is mounted external to a section of flue pipe or is an integral part of a section of flue pipe. The heat exchanger preheats a domestic hot water supply and boosts the return water temperature prior to reentry to the furnace coil. The heat exchanger reduces fuel use, pollution and wear of the furnace and burner. A typical heat exchanger installation includes an oil or gas burner located on a furnace or boiler having a flue pipe leading to a gaseous outlet, such as a masonry chimney. A short vertical flue section leads to a draft-regulating damper. The flue heat exchanger may be a coil of copper tubing wrapped around flue section, such that the tubing picks up heat from the heated flue gasses. The cold water source is coupled to a short length of convoluted flexible tubing with coupling flanges which couple the water through the flue mounted heat exchanger. Preheated water exits from the heat exchanger through flexible tubing having a safety pressure relief valve.

FIELD OF THE INVENTION

[0001] The present invention relates to heat exchangers, which aremounted external to a section of flue pipe or are an integral part of asection of flue pipe.

BACKGROUND OF THE INVENTION

[0002] Heat exchangers are known, which direct water in a pipe through aflue.

[0003] U.S. Pat. No. 4,122,801 of Burns describes a heat exchanger whichfirst encircles water pipes in 2 circular rings around an exhaust flue,but then directs the water into coils within the exhaust flue. Theexternal rings appear to be for positioning the water flow headersequally around the flue, not to preheat the water.

[0004] U.S. Pat. No. 4,211,187 of Farris discloses an energyconservation system for heaters that uses a heat exchanger in a furnacechamber or duct.

[0005] U.S. Pat. No. 4,136,731 of DeBoer discloses a heat transferapparatus for supplementing a building heating and cooling system, usinga heat exchanger in a furnace flue. DeBoer suggests an external heatexchanger, but provides no enabling details thereof.

[0006] U.S. Pat. No. 4,484,564 of Erickson disclose a water heaterutilizing exhaust gases from furnaces or stoves, but the recovery isthrough a coil inside an exhaust flue.

[0007] U.S. Pat. No. 1,990,056 of Van Daam describes passing waterthrough a spherical corrugated chamber.

[0008] U.S. Pat. No. 3,896,992 of Borovina and 2,521,462 of Kinzelmannboth disclose water heaters that pass water through a spiral coil withinan exhaust flue.

[0009] U.S. Pat. No. 4,037,567 of Torres proposes an exhaust flue overthe water heater having a spiral coil for heating water therein.

[0010] U.S. Pat. No. 4,120,267 of Wood describe a tube and plate heatexchanger with water heating coils inside a chamber, such as a gas heatduct or flue.

[0011] Furthermore, U.S. Pat. No, 4,401,261 of Brown also disclosesdirecting water coils inside of flues.

OBJECTS OF THE INVENTION

[0012] It is therefore an object of the present invention to provide aheater exchanger for domestic hot water with ease of installation,maintenance and removal.

[0013] It is a further object of the present invention to preheat adomestic hot water supply.

[0014] It is yet another object of the present invention to boost returnwater temperature in a hydronic heating installation prior to reentry toa furnace coil.

[0015] It is still another object of the present invention to reducefuel use and to reduce pollution and wear of the furnace and burner.

[0016] It is yet another object of the present invention to improve overthe disadvantages of the prior art.

SUMMARY OF THE INVENTION

[0017] In keeping with these objects and others, which may becomeapparent, the present invention includes heat exchangers mountedexternal to a section of flue pipe or as an integral part of a sectionof flue pipe. The heat exchanger preheats the domestic hot water supplyand boosts the return water temperature in a hydronic heatinginstallation prior to reentry to the furnace coil. The heat exchangerreduces fuel use and reduces pollution and wear of the furnace andburner.

[0018] A typical heat exchanger installation includes an oil or gasburner located on a furnace or boiler having a flue pipe leading to agaseous outlet, such as a masonry chimney. A short vertical flue sectionleads to a draft-regulating damper. The flue heat exchanger may be acoil of copper tubing wrapped around the flue section, which picks upheat from the heated flue gasses. The cold water source is coupled to ashort length of convoluted flexible tubing with coupling flanges therebyallowing water to travel to and from the flue mounted heat exchanger.

[0019] In another embodiment two flue heat exchangers communicate withcold water entering the horizontal heat exchanger which is wrappedaround a flue section having a mixture of hot flue gasses and somemake-up ambient air from the draft regulating damper. This heatexchanger is plumbed in series with another heat exchanger wrappedaround the vertical section of flue pipe below a damper. The second heatexchanger again increases the water temperature prior to entering thefurnace 5 hot water coil.

[0020] In another embodiment a heat exchanger is prefabricated as astandard flue section and substitutes for a length of flue. This heatexchanger has a central flue pipe section with heat exchanger tubingwrapped around its periphery. A tubular shell encases the tubing withopenings allowing for both the water inlet and water outlet couplingflanges. A highly conductive conformable material fills the empty spaceswithin the shell to increase heat transfer.

[0021] In another embodiment, a hydronic heating system is a hydronicloop circulated by circulator pump forcing water into heating coilinside a boiler or furnace. In this system, it first flows through heatexchanger where it picks up waste heat from the flue.

[0022] In a further preferred embodiment a preformed heat exchanger coilis wrapped around a cylindrically shaped sheet larger in diameter than asection of flue. This sheet is not totally enclosed, but it has a smallgap along its length.

[0023] In yet another embodiment, two coiled heat exchanger conduits areinterleaved together around a flue pipe.

[0024] Each embodiment of the heat exchanger may include a safetypressure relief valve through which preheated water may exit.

[0025] Often when two or more different types of metals contact eachother, the metals deteriorate, corrode or weaken at the point ofcontact. Therefore, each embodiment of flue heat exchanger may also beconstructed such that both the metal tubing and the section of fluepiping that the metal tubing is wrapped around are made from the sametype of metal, preferably copper. In addition, each embodiment thatcontains this variation may also include gaskets located at each end ofthis flue section (preferably copper), such that the gaskets preventdirect contact between this flue pipe section and the flue pipe sectionmade from a different type of metal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The present invention can best be understood in conjunction withthe accompanying drawings, in which:

[0027]FIG. 1 is a side elevation view of a flue heat exchangerinstallation of the present invention;

[0028]FIG. 2 is a side elevation view of an installation using two flueheat exchangers;

[0029]FIG. 3 is a perspective view of an alternate embodiment for a flueheat exchanger;

[0030]FIG. 3a is a perspective view of an alternate embodiment of a heatexchanger cover;

[0031]FIG. 4 is a side cross sectional view of the embodiment shown inFIG. 3;

[0032]FIG. 5 is a plumbing diagram of a flue heat exchanger used forhydronic heating;

[0033]FIG. 6 is a side elevation view of a preferred embodiment of aflue heat exchanger;

[0034]FIG. 7 is an end view of the embodiment shown in FIG. 6;

[0035]FIG. 8 is a side elevation view of an interleaved heat exchangerinstallation;

[0036]FIG. 9 is a perspective view of another embodiment of a flue heatexchanger; and

[0037]FIGS. 9A and 9B are respective end and cross sectional views ofthe embodiment shown in FIG. 9.

DETAILED DESCRIPTION OF THE INVENTION

[0038] For ease of installation, maintenance, and removal, the heatexchangers of this invention are mounted external to a section of fluepipe or are an integral part of a section of flue pipe.

[0039] Although the primary application is the preheating of a domestichot water supply, a secondary application is the boosting of returnwater temperature in a hydronic heating installation prior to reentry tothe furnace coil. In either case, the objectives are to reduce fuel useand to reduce pollution and wear of the furnace and burner.

[0040]FIG. 1 shows a typical installation showing an oil or gas burner 6on a furnace or boiler 5 with flue pipe section 2 leading to masonrychimney 3. A short vertical flue section 15 leads to a draft regulatingdamper 4. The flue heat exchanger 1 includes a coil of heat conductivetubing, such as copper tubing, which is wrapped around flue section 2.Flue heat exchanger picks up heat from the heated flue gasses withinflue section 2. The cold water source 7 is coupled to a short length ofconvoluted flexible tubing 9 with coupling flanges 8 at either end,which couple the cold water through heat exchanger 1. The exit ofpreheated water from heat exchanger 1 is coupled to another short lengthof flexible tubing 9 and then coupled to a short length of pipe viacoupling flanges 8.

[0041] This leads to a safety pressure relief valve 10 and isolationvalve 11 (normally open) which couples the preheated water to a domesticsupply pipe 12 (optional) and to the boiler 5 hot water coil intake 13.Pipe 14 is the normal domestic hot water supply line from the hot watercoil.

[0042] The need for safety pressure relief valve 10 is predicated onrare events, which could conspire to cause boiling or excessive pressurein heat exchanger 1. While flue surface temperatures above 212 degreesF. are sometimes encountered, water at normal supply pressure (above 35psig) requires a flue temperature over 280 degrees F. to present adanger of boiling. In the event of a defective well pump or control, thewater pressure could be abnormally low. Likewise, an improperly adjustedfuel burner could produce abnormally high flue temperatures approaching300 degrees F. The combination of such events may result in excessivepressures, hence the pressure relief valve. This danger is morepronounced in situations with a common boiler supplying heat and hotwater since it would be more likely for the burner to be on while thereis no call for domestic hot water; and there would be no cooling waterflow through the heat exchanger.

[0043]FIG. 2 shows an installation using two flue heat exchangers 1 and20 in a single installation. The coldest water enters the horizontalheat exchanger 1, which is wrapped around flue section 2 having amixture of hot flue gasses and some make-up ambient air from draftregulating damper 4. Heat exchanger 1 is plumbed in series with heatexchanger 20, which is wrapped around the vertical section 2 a of fluepipe below damper 4. This section of flue pipe 2 a has a surfacetemperature higher than flue section 2 since it is closer to furnace 5and only has hot flue gasses within. Therefore, heat exchanger 20further boosts water temperature prior to entering the furnace 5 hotwater coil.

[0044]FIGS. 3 and 4 show two views of an alternate embodiment 25 of theheat exchanger of this invention. This heat exchanger 25 isprefabricated as a standard flue section length L. It would be simplysubstituted for a length of flue. Heat exchanger 25 includes a centralflue pipe section 26 with heat exchanger tubing 30 wrapped around itsperiphery. A tubular shell 31 with heat resistant end caps 32 encasesthe tubing 30 with openings for inlet 28 and outlet 27 extensionsterminating in coupling flanges 29. A highly conductive conformablematerial 33 such as copper or aluminum wool is forced between flue 26surface and coils 30 and generally fills the empty spaces within shell31 to increase heat transfer. Further efficiency is achieved if shell 31is a thermal insulator such as a fiberglass liner within a plastic hardshell.

[0045]FIG. 3a shows a removable embodiment of tubular shell 31 withopenings for inlet 28 and outlet 27 extensions. A highly heat conductiveconformable material 33 such as copper or aluminum wool is secured tothe entire inner wall of tubular shell 31. Clasps 80 are placed alongthe outside of the surface of tubular shell 31 thereby allowing a meansto secure the tubular shell around both flue pipe 26 and heat exchangercoil 30.

[0046]FIG. 5 shows the plumbing hook-up for the use of an external flueheat exchanger 1 in a hydronic heating system using fin tube heatingelements 44. The basic circuit is a hydronic loop circulated bycirculator pump 40 forcing water into heating coil 41 inside boiler(furnace) 5 then leading to expansion tank 42 and further to a parallelarrangement of zone valves 43 through fin tube sections (baseboard hotwater room units) 44 and through return manifold 45. In systems withoutheat exchanger 1, this return flow would be directly plumbed to theintake of circulator pump 40. In this system, the return flow firstflows through heat exchanger 1 where it picks up waste heat from theflue.

[0047] The preferred embodiment shown in FIGS. 6 and 7 shows a preformedheat exchanger coil 1 wrapped around a cylindrically shaped sheet (orsleeve) of metal 55 such as copper which is slightly larger in diameterthan a section of flue. Sheet 55 is not totally enclosed, but it has asmall gap along its length. The cylindrically shaped sheet is solderedor brazed to the copper tubing 1 for approximately three-quarters of itscircumference to enhance heat transfer. The region indicated by 90degrees in FIG. 7 is not bonded to tubing 1, thereby allowing the sleeveto open and enclose the flue. Three sets of clamping tabs 58 are locatedat each end and in the middle in a gap between adjacent coils of tubing1. Bolts 56 and nuts 57 are used through tabs 58 to insure sheet 55 fitssecurely around a flue section 2.

[0048] An alternate embodiment specifically for dual-use boilerinstallations is shown in FIG. 8. An interleaved coil flue heatexchanger 70 is shown wrapped over flue pipe section 2. It consists oftwo separate conduits. Coil 71 plumbed into the domestic hot waterreturn 13 and coil 72 plumbed into the hydronic heating return line 73.The method for forming this flue heat exchanger 70 is to coil twolengths of bendable tubing together resulting in the interleaved coilsof sections 71 and 72. The plumbing is straightforward with the outletend of coil 72 leading to circulator pump 40 through conduit 74 thusboosting the temperature of heating water that has been cooled by itsflow through the various room hydronic heating units such as baseboardfin tube units or radiators. Similarly, the cold supply water at 7 usesthe interleaved flue heat exchanger 70, a single section of flue pipe 2can be used to recover waste heat year round regardless of whether theburner 6 is being fired to generate heat, hot water, or both. It ishighly likely that circulator pump 40 will be running or that domestichot water demand will occur while burner 6 is active or while flue pipe2 is still hot from a recent firing. For dual-use installations, thismaximizes the flue waste heat recover on a seasonal basis.

[0049]FIGS. 9, 9A and 9B show an alternate embodiment of a flue heatexchanger with particular gaskets 85, 86. This is because when two ormore different types of metals contact each other, the metals maysometimes deteriorate, corrode or weaken at the point of contact.Therefore, while it is preferable that a flue heat exchanger may beconstructed such that both the metal tubing 88 and the section 87 offlue piping that the metal tubing 88 is wrapped around are made from thesame type of metal, preferably copper, modifications must be made ifdifferent metals are in contact with each other.

[0050] Therefore, FIGS. 9, 9A and 9B show a flue piping section 87 of aflue heat exchanger having metal tubing 88 wrapped around it, with fluepiping section 87 having gaskets 85, 86 located at each end of fluepiping section 87(preferably copper), such that the gaskets 85, 86prevent direct contact between this flue pipe section 87 and thepermanent flue pipe section which may be made from a different type ofmetal other than copper, to which flue pipe section 87 is attached.

[0051] It is further noted that other modifications may be made to thepresent invention, without departing from the scope of the invention, asnoted in the appended Claims.

I claim:
 1. A heat exchanger for preheating domestic water comprising ahollow tubing having a water inlet end and a water outlet end, saidhollow tubing external to and wrapped around a section of a flue pipe ofa furnace.
 2. The heat exchanger as in claim 1 further comprising apressure relief valve coupling one end of said relief valve to the wateroutlet end of said hollow tubing.
 3. The heat exchanger as in claim 2further comprising an isolation valve coupling said relief valve to aboiler hot water coil intake.
 4. The heat exchange as in claim 2 furthercomprising an automatic feeder coupling said relief valve to a boilerhot water coil intake.
 5. The heat exchanger as in claim 1 wherein saidheat exchanger further comprising a second heat exchanger connected inseries therewith and wrapped around and external to another section offlue piping, further increasing water temperature prior to entering thefurnace hot water coil.
 6. The heat exchanger as in claim 1 wherein saidheat exchanger is prefabricated as a standard flue section, whichsubstitutes for a length of said flue pipe, said pre-fabricated heatexchanger having a central flue pipe section with heat exchanger tubingwrapped around its periphery, said pre-fabricated heat exchanger furtherhaving encasement over said heat exchanger tubing.
 7. The heat exchangeras in claim 6, wherein said encasement permanently encloses said tubing,said encasement has a cavity and heat resistant end caps, saidencasement also having a plurality of openings sufficient for a wateroutlet and water inlet to protrude therefrom, said cavity having ahighly heat conductive material packed therein.
 8. The heat exchanger asin claim 6, wherein said encasement comprises a preformed semi-pliablesleeve, said sleeve having two edges having clasps that when broughttogether close said sleeve and enclose said tubing forming a pluralityof holes sufficient for said water inlet and said water outlet toprotrude therefrom.
 9. The heat exchanger as in claim 7 wherein saidencasement includes a thermal insulator between said encasement's innersurface and the heat conductive material.
 10. The heat exchanger as inclaim 9 wherein said thermal insulator is a fiberglass liner.
 11. Theheat exchanger as in claim 1 wherein said heat exchanger is a hydronicheating system using fin tube heating elements, said heat exchangerhaving a hydronic loop circulated by a circulator pump forcing waterinto a heating coil inside the boiler then leading to an expansion tankand further to an arrangement of zone valves through fin tube baseboardhot water room sections and then through a return manifold.
 12. The heatexchanger as in claim 1 wherein said heat exchanger coil is wrappedaround a cylindrically shaped sheet of conductive metal, said cylinderhaving three quarters of its circumference affixed to a correspondingarea of coil, which said sheet is slightly larger in diameter than asection of said flue pipe, said sheet having a small gap along itslength, said gap allowing said flue section to slide into said cylinder,said cylinder further comprising a locking means to secure said cylinderwhen said gap is closed.
 13. The heat exchanger as in claim 12 whereinsaid cylindrical sheet is copper, said cylinder also having gaskets onthe inner and outer periphery of said cylinder sufficient in size toprevent contact with flue pipe sections made from a different type ofmetal.
 14. The heat exchanger as in claim 1 further comprising a pair ofinterleaved flue heat exchanger coils wrapped together around said fluepipe, said coils comprising two separate conduits for separate transferof heated water therethrough.
 15. The heat exchanger for a domesticsource of heat having a flue pipe leading to a masonry chimney,comprising: a coil of heat conductive tubing wrapped around a section ofsaid flue, said coil picking up heat from the heated flue gasses whereinsaid flue section is made of copper, said flue section further havinggaskets located on respective inner and an outer edge peripheries,thereby preventing direct contact with flue sections made from metalother than copper.
 16. The heat exchanger as in claim 15 wherein a coldwater source is coupled to a length of convoluted flexible tubing havingcoupling flanges at either end, wherein said coupling flanges allowwater transport to and from said heat exchanger.
 17. The heat exchangeras in claim 16 wherein the preheated water from said heat exchanger iscoupled to a boiler hot water coil intake.
 18. The heat exchanger as inclaim 17, wherein the preheated water from said heat exchanger is alsocoupled to a domestic supply pipe.
 19. The heat exchanger as in claim 15further comprising a pressure relief valve.
 20. A heat exchanger forpreheating domestic water comprising a hollow tubing having a waterinlet end and a water outlet end, said hollow tubing external to andwrapped around a section of a flue pipe of a furnace, said heatexchanger is prefabricated as a standard flue section, which substitutesfor a length of said flue pipe, said pre-fabricated heat exchangerhaving a central flue pipe section with heat exchanger tubing wrappedaround its periphery, said pre-fabricated heat exchanger further havingan encasement over said heat exchanger tubing, said encasement enclosingsaid tubing, said encasement having a cavity and heat resistant endcaps, said encasement also having a plurality of openings sufficient fora water outlet and water inlet to protrude therefrom, said cavity havinga highly heat conductive material packed therein, said encasementcomprising a preformed semi-pliable sleeve, said sleeve having two edgeshaving clasps that when brought together close said sleeve and enclosesaid tubing forming a plurality of holes sufficient for said water inletand said water outlet to protrude therefrom, and, said encasementincluding a thermal insulator between said encasement's inner surfaceand the heat conductive material.